Thin film transistor for supplying power to element to be driven

ABSTRACT

One or more element driving TFTs ( 20 ) for controlling the amount of current supplied from a power supply line VL is (are) provided between an organic EL element ( 50 ) and the power supply line VL. The element driving TFT ( 20 ) is placed so that its channel length direction is parallel to the longitudinal direction of the pixel, the extension direction of a data line for supplying a data signal to a switching TFT ( 10 ) for controlling the element driving TFT ( 20 ), or the scan direction of laser annealing for polycrystallizing the active layer ( 16 ) of the TFT ( 20 ).

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an electroluminescent displaydevice, and in particular, to transistors constructing the circuitstructure in the pixel section of an electroluminescent display device.

[0003] 2. Description of the Related Art

[0004] An electroluminescence (hereinafter referred to as EL) displaydevice which uses an EL element which is a self-illuminating element asan illumination element in each pixel has attracted a strong interest asan alternative display device for a display device such as a liquidcrystal display device (LCD) and a CRT because the EL display device hasadvantages such as thin width and low power consumption, in addition tothe advantage of being self-illuminating. Such an EL display device hasthus been researched.

[0005] In particular, there is a high expectation for an active matrixtype EL display device in which a switching element such as, forexample, a thin film transistor for individually controlling an ELelement is provided for each pixel and EL elements are controlled foreach pixel, as a high resolution display device.

[0006]FIG. 1 shows a circuit structure for one pixel in an active matrixtype EL display device having m rows and n columns. In the EL displaydevice, a plurality of gate lines GL extend on a substrate in the rowdirection and a plurality of data lines DL and power supply lines VLextend on the substrate in the column direction. Each pixel has anorganic EL element 50, a switching TFT (first TFT) 10, an EL elementdriving TFT (second TFT) 20, and a storage capacitor Cs.

[0007] The first TFT 10 is connected to the gate line GL and data lineDL, and is turned on by receiving a gate signal (selection signal) onits gate electrode. A data signal which is being supplied on the dataline DL at this point is then held in the storage capacitor Cs connectedbetween the first TFT 10 and the second TFT 20. A voltage correspondingto the data signal is supplied to the gate electrode of the second TFT20 via the first TFT 10. The second TFT 20 then supplies a current,corresponding to the voltage value, from the power supply line VL to theorganic EL element 50. In this manner, the organic EL element in eachpixel is illuminated at a brightness based on the data signal, and adesired image is displayed.

[0008] The organic EL element is a current-driven element which isilluminated by supplying a current to an organic emissive layer providedbetween a cathode and an anode. The data signal output onto the dataline DL, on the other hand, is a voltage signal with an amplitudecorresponding to the display data. Thus, conventionally, in order toaccurately illuminate the organic EL element by such a data signal, inan organic EL display device, a first TFT 10 and a second TFT 20 areprovided in each pixel.

[0009] The display quality and reliability of the organic EL displaydevices described above remain insufficient, and the characteristicvariations in the first and second TFTs 10 and 20 must be dissolved. Inparticular, reduction in characteristic variation in the second TFT 20for controlling the amount of current supplied from the power supplyline VL to the organic EL element 50 is desired, because such variationdirectly causes variation in the illumination brightness.

[0010] Moreover, it is preferable to construct the first and second TFTs10 and 20 from a polycrystalline silicon TFT which has quick operationspeed and which can be driven by a low voltage. In order to obtain apolycrystalline silicon, an amorphous silicon is polycrystallized bylaser annealing. Because of various reasons such as, for example, energyvariation in the irradiating laser at the irradiation surface, the grainsize of the polycrystalline silicon is not uniform. When grain size isnot uniform, in particular around the TFT channel, there is a problem inthat the on-current characteristic or the like of the TFT may also vary.

SUMMARY OF THE INVENTION

[0011] The present invention is conceived to solve the above problem,and one object of the present invention is to provide an active matrixtype organic EL panel capable of illuminating each illumination pixel ata uniform brightness by alleviating the characteristic variations of theTFT which controls the organic EL element.

[0012] According to one aspect of the present invention, there isprovided an active matrix type display device in which each of aplurality of pixels arranged in a matrix form comprises at least anelement to be driven and an element driving thin film transistor forsupplying power from a driving power supply to the element to be driven;wherein each pixel region of the plurality of pixels has one of thesides in the row direction or column direction of the matrix longer thanthe other side; and the element driving thin film transistor is placedso that its channel length direction is along the longer side of thepixel region.

[0013] According to another aspect of the present invention, in thedisplay device, it is preferable that in the pixel region the side alongthe column direction of the matrix is longer than the side along the rowdirection of the matrix; and that the element driving thin filmtransistor is placed so that its channel length direction is along thecolumn direction.

[0014] According to another aspect of the present invention, there isprovided a semiconductor device comprising at least one element drivingthin film transistor for supplying driving current from a power supplyline to a corresponding element to be driven; and a switching thin filmtransistor for controlling the element driving thin film transistorbased on data supplied when selected; wherein the element driving thinfilm transistor is placed so that its channel length direction is alongthe extension direction of a data line for supplying the data signal tothe switching thin film transistor.

[0015] By employing such a configuration, it is possible to increase thechannel length of the element driving thin film transistor for supplyingpower to the element to be driven, and, to thereby improve reliabilitycharacteristics of the transistor such as, for example, its durability.In addition, the characteristic of the element driving thin filmtransistors each provided for an element to be driven can be averaged,and, thus, the variation in the illumination brightness among theelements can be inhibited even when the element to be driven is anemissive element which has different illumination brightness dependingon the supplied power. Moreover, the configuration facilitates efficientplacement of a plurality of element driving thin film transistors, forexample, with sufficient channel length with respect to one element tobe driven, in parallel or in series within a pixel and, thus, it ispossible to increase the illumination region in a case where the elementto be driven is an emissive element.

[0016] According to another aspect of the present invention, in thesemiconductor device or display device, it is preferable that theelement driving thin film transistor is formed so that its channellength direction is along the scan direction of a line pulse laser forannealing the channel region of the transistor.

[0017] In this manner, by coinciding the channel length direction of theelement driving thin film transistor and the scan direction of the laserannealing, the difference in the transistor characteristics from theelement driving thin film transistors for supplying power to otherelements to be driven can be reliably reduced.

[0018] In laser annealing, the laser output energy tends to vary. Thevariation includes a variation in the pulse laser within an irradiationregion and variation among the shots. In many cases, the element drivingthin film transistor which is used for a semiconductor device such as,for example, an active matrix type display device, is designed so thatthe channel length is significantly greater than the channel width. Byplacing the element driving thin film transistor along the longer sideof the pixel region or forming the element driving thin film transistoralong the column direction or the extension direction of the data line,the channel length of the element driving thin film transistor caneasily be increased to a sufficient length. By setting the scandirection of the laser to almost coincide with the channel lengthdirection of the element driving thin film transistor, that is, bysetting the scan direction of the laser so that the longitudinaldirection of the laser irradiation region crosses the channel in thewidth direction, the device can easily be adjusted so that the entirechannel region of one element driving thin film transistor is notsimultaneously annealed by a single shot. This can be easily achievedby, for example, setting the channel length of the element driving thinfilm transistor to be longer than one moving pitch of a pulse laser.Thus, in a case where a plurality of elements to be driven is formed onthe same substrate and a plurality of element driving thin filmtransistors for supplying power to the plurality of elements is formed,it is possible to laser anneal the active layers of the thin filmtransistors by a plurality of shots, resulting in the transistors to besubjected more uniformly to the energy variation among the shots andreliable averaging of the characteristics among the thin filmtransistors. In this manner, for example, in an organic EL displaydevice which uses an organic EL element as the element to be driven,which uses an organic compound as the emissive layer, variation in theillumination brightness among the organic EL elements provided fordifferent pixels can be significantly reduced.

[0019] According to another aspect of the present invention, in thesemiconductor device, it is preferable that the channel length directionof the element driving thin film transistor does not coincide with thechannel length direction of the switching thin film transistor.

[0020] The switching thin film transistor is placed near the sectionwhere the selection line for selecting the transistor and the data linefor supplying a data signal cross each other. In many cases, theswitching thin film transistor is placed so that its channel lengthdirection is approximately parallel to the extension direction of theselection line. In such a case, by placing the element driving thin filmtransistor so that its channel length direction is different from thatof the switching thin film transistor, the channel length of the elementdriving thin film transistor can easily be increased.

[0021] According to another aspect of the present invention, it ispreferable that the element to be driven is an organicelectroluminescence element which employs an organic compound as anemissive layer. Although such an organic EL element has high brightnessand wider selection ranges for the illumination color and material,because the organic EL element is current driven, variation in theamount of supplied current causes a variation in the illuminationbrightness. By using the circuit structure of the pixel or placement asdescribed above, it is possible to easily maintain uniformity of thesupplied current. In addition, by employing the placement and structureof the contact points as described above, the aperture ratio can beincreased and the element layer such as the emissive layer can be formedon a flat surface, and a more reliable element can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a diagram showing a circuit structure for one pixel inan active matrix type organic EL display device.

[0023]FIG. 2 is a diagram showing an example circuit structure of onepixel in an active matrix type organic EL display device according to afirst embodiment of the invention.

[0024]FIG. 3 is a diagram showing the I-V characteristic of a TFT.

[0025]FIGS. 4A and 4B are diagrams showing the effects obtained by thecircuit structure of the present invention and by a circuit ofconventional structure.

[0026]FIG. 5 is a diagram showing another circuit structure of one pixelin an active matrix type organic EL display device according to thefirst embodiment of the invention.

[0027]FIG. 6 is a diagram showing yet another circuit structure of onepixel in an active matrix type organic EL display device according tothe first embodiment of the invention.

[0028]FIG. 7 is a diagram showing still another circuit structure of onepixel in an active matrix type organic EL display device according tothe first embodiment of the invention.

[0029]FIG. 8 is a diagram showing planer structure of the active matrixtype organic EL panel according to the first embodiment of the presentinvention with the circuit structure shown in FIG. 7.

[0030]FIGS. 9A, 9B, and 9C are diagrams respectively showing the crosssectional structure along the lines A-A, B-B, and C-C of FIG. 8.

[0031]FIGS. 10A and 10B are respectively a planer diagram and a crosssectional diagram of one pixel of the active matrix type organic ELpanel according to a second embodiment of the present invention.

[0032]FIG. 11 shows another example of a planer structure of one pixelof the active matrix type organic EL panel according to the secondembodiment.

[0033]FIG. 12 is a planer diagram of one pixel of the active matrix typeorganic EL panel according to a third embodiment of the presentinvention.

[0034]FIG. 13 shows another example of a planer structure of one pixelof the active matrix type organic EL element according to the thirdembodiment.

[0035]FIG. 14 shows yet another example of a planer structure of onepixel of the active matrix type organic EL panel according to the thirdembodiment.

[0036]FIGS. 15A and 15B are diagrams respectively showing the crosssectional structure and planer structure of the contact section betweenthe active layer 16 of the second TFT and the anode 52 of the organic ELelement 50.

[0037]FIGS. 16A and 16B are diagrams respectively showing the crosssectional structure and planer structure of the contact section betweenthe active layer 16 of the second TFT and the anode 52 of the organic ELelement 50 according to a fourth embodiment of the present invention.

[0038]FIG. 17 is a diagram showing another example of a cross sectionalstructure of the contact section between the active layer 16 of thesecond TFT and the anode 52 of the organic EL element 50 according tothe fourth embodiment.

[0039]FIG. 18 is a diagram showing a further example of a crosssectional structure of the contact section between the active layer 16of the second TFT and the anode 52 of the organic EL element 50according to the fourth embodiment.

[0040]FIG. 19 is a diagram showing yet another example of a crosssectional structure of the contact section between the active layer 16of the second TFT and the anode 52 of the organic EL element 50according to the fourth embodiment.

[0041]FIG. 20 is diagram showing another example of a cross sectionalstructure of the contact section between the active layer 16 of thesecond TFT and the anode 52 of the organic EL element 50 according tothe third embodiment.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0042] Preferred embodiments of the present invention (hereinafterreferred to simply as embodiments) will now be described referring tothe drawings.

First Embodiment

[0043]FIG. 2 shows a circuit structure of one pixel in an active matrixtype EL display device having m rows and n columns according to a firstembodiment of the present invention. As shown in FIG. 2, each pixelcomprises an organic EL element 50, a switching TFT (first TFT) 10, anelement driving TFT (second TFT) 20, and an storage capacitor Cs, and isconstructed in a region surrounded by a gate line GL extending in therow direction and a data line DL extending in the column direction. Inthe first embodiment, a compensation TFT 30 having the conductivecharacteristic opposite of that of the second TFT 20 is provided betweenthe power supply line VL and the second TFT 20. The gate and either thesource or the drain of the compensation TFT 30 are connected to providea diode connection. The diode is connected in the forward directionbetween the power supply line VL and the second TFT 20. Thus, thecompensation TFT can be operated without supplying any designatedcontrol signal.

[0044] The first TFT 10 is turned on by receiving a gate signal at itsgate. When the first TFT 10 is turned on, the data signal supplied tothe data line DL is held at the storage capacitor Cs connected betweenthe first and second TFTs 10 and 20, and the potential at one electrodeof the storage capacitor Cs becomes equal to the data signal. The secondTFT 20 is provided between the power supply line VL and the organic ELelement (the anode of the element) 50, and operates to supply a currentcorresponding to the voltage value of the data signal applied to itsgate, from the power supply line VL to the organic EL element 50. In theexample shown in FIG. 2, an nch-TFT which is capable of high-speedresponse is used for the first TFT 10 and a pch-TFT is used for thesecond TFT 20.

[0045] An nch-TFT having a polarity opposite that of the second TFT 20is used for the compensation TFT 30. When the I-V (current-voltage)characteristic of the second TFT 20 is varied, the I-V characteristic ofthe compensation TFT 30 is varied in the opposite direction, thuscompensating for the characteristic variation of the second TFT 20.

[0046]FIG. 3 shows I-V characteristics of an nch-TFT and a pch-TFT whichuse polycrystalline silicon for the active layer. In the nch-TFT, whenthe voltage applied to the gate exceeds a predetermined positive voltage(+Vth), the current value is rapidly increased. In the pch-TFT, on theother hand, when the voltage applied to the gate becomes less than apredetermined negative voltage (−Vth), the current value is rapidlyincreased. In an nch-TFT and a pch-TFT formed on the same substrate, forexample, when the threshold value, +Vth, of the nch-TFT is varied by anincrease, that is, shifted to the right in FIG. 3, the threshold value,−Vth, of the pch-TFT is shifted by about the same degree to the right ofFIG. 3. In contrast, when the threshold value, +Vth, of the nch-TFT isshifted to the left, the threshold value, −Vth, of the pch-TFT is alsoshifted to the left. For example, due to a variation in themanufacturing condition or the like, when −Vth of the pch-TFT used forthe second TFT 20 of FIG. 2 is shifted to the right, in a conventionaldevice, the amount of current supplied to the organic EL element 50under the same condition would immediately be reduced. However,according to the first embodiment, the amount of current flowing fromthe compensation TFT 30, which is provided between the second TFT 20 andthe power supply line VL and which is constructed from an nch-TFT, isincreased.

[0047] According to the first embodiment, as shown in FIG. 2, a secondTFT 20 and a compensation TFT 30 of opposite polarity are providedbetween the power supply line VL and the organic EL element 50. The twoTFTs are thus balanced. That is, the amounts of current flowing fromthese TFTs always compensate for each other. In the circuit structure ofthe first embodiment, due to the presence of the compensation TFT 30,the maximum current value which can be supplied to the organic RLelement 50 is less than that in the conventional circuit structure shownin FIG. 1 which does not have the compensation TFT 30. However, becausethe identification sensitivity of human eyes at a high brightness issignificantly lower than that at an intermediate brightness, a smallreduction in the maximum current value which can be supplied does notsignificantly influence the display quality. On the other hand, becausethe second TFT 20 and the compensation TFT 30 adjust the current flowingfrom each other in each pixel, the variation in the amount of currentsupplied to the organic EL element 50 among the pixels can be reduced.

[0048] Now referring to FIGS. 4A and 4B, an advantage obtained by thecircuit structure of the first embodiment will be described. FIG. 4Ashows the relationship between the applied voltage (data signal) and theillumination brightness for an example case where the organic EL elementis illuminated by the pixel circuit structure of the first embodimentshown in FIG. 2. Similarly, FIG. 4B shows the relationship between theapplied voltage (data signal) and the illumination brightness for anexample case wherein the organic EL element is illuminated by theconventional pixel circuit structure shown in FIG. 1. The setting inboth FIGS. 4A and 4B is such that the requested maximum brightness withrespect to the organic EL element occurs at the applied voltage (datasignal) of 8V, and the gradation display is performed at an appliedvoltage between 8V and 10V. The three samples in FIGS. 4A and 4Brespectively indicate illumination brightness characteristics for caseswherein organic EL panels having circuit structure respectively of FIG.2 and FIG. 1 are formed under different manufacturing conditions. Inother words, these samples indicate illumination brightnesscharacteristics for cases where the characteristic of the TFT in thepixel section is deliberately varied.

[0049] As is clear from FIGS. 4A and 4B, with the conventional circuitstructure, the brightness (luminance) characteristics for the threesamples having different characteristics for TFT in the pixel sectionsignificantly differ from each other at the set voltage range for datasignals between 8V and 10V. In contrast, with the circuit structureaccording to the first embodiment, although the characteristics differfrom each other at the high-brightness region to which human eyes areinsensitive, the brightness characteristic difference among the threesamples at the intermediate-brightness region is very small. Therefore,by employing a circuit structure as described in the first embodimentfor each pixel, even when the characteristic of the TFT is varied, inparticular, even when the characteristic of the EL element driving TFT20 which has a large influence is varied, the variation can becompensated by the compensation TFT 30 of an opposite polarity, thusenabling inhibition of the variation in the illumination brightness ofthe organic EL element.

[0050]FIG. 5 shows another example circuit structure according to thefirst embodiment. The circuit structure of FIG. 5 differs from that ofFIG. 2 in that the second TFT 22 is constructed using an nch-TFT and thecompensation TFT 32 is constructed using a diode connected pch-TFT.Similar to the above structure, with this structure, the variation incharacteristic of the second TFT 22 can be compensated by thecompensation TFT 32.

[0051]FIG. 6 shows yet another example of a circuit structure accordingto the first embodiment. The circuit structure of FIG. 6 differs fromthat of FIG. 2 in that a plurality of second TFTs are provided inparallel between the compensation TFT 30 and the organic EL element 50.The polarity of the TFTs are identical to that in FIG. 2, that is, thesecond TFTs 24 are of pch and the compensation TFT 30 is of nch. Thegates of two second TFTs 24 are both connected to the first TFT 10 andto the first electrode of the storage capacitor Cs. Each of the sourcesis connected to the compensation TFT 30 and each of the drains isconnected to the organic EL element 50. In this manner, by providing aplurality of the second TFTs 24 in parallel, it is possible to furtherreduce the variation in the current supplied to the organic EL elementdue to the characteristic variation of the second TFT.

[0052] If the target current value for each of the two second TFTs 24 isassumed to be i, the total target current value for the two second TFTs24 would be 2 i. Even when the current supply capability for one of thesecond TFTs 24 is reduced to i/2 due to, for example, variations, theother second TFT 24 can continue to flow a current of i, and a totalcurrent of (3/2)i can be supplied to the organic EL element, where thetarget value is 2 i. In the worst case, if the current supply capabilityof one of the TFTs becomes 0, a current of i can be supplied to theorganic EL element by the other TFT in the example shown in FIG. 6. Theadvantage of such a structure can be readily seen when one considers acase wherein the current supply capability becomes 0 in a circuit havingsingle second TFT 24 and the pixel becomes deficient.

[0053] Each TFT in the first embodiment is obtained by polycrystallizingan a-Si by a laser annealing process. When a plurality of second TFTs 24are provided in parallel, it is easy to arrange the positions of thesecond TFTS 24 so that the laser is not simultaneously irradiated toactive layers of both second TFTs 24 by, for example, shifting theformation positions with respect to the laser scan direction. Byarranging the second TFTs 24 in this manner, the probability that allsecond TFTs 24 become deficient can be significantly reduced, and thus,characteristic variation caused by the laser annealing can be minimized.In addition, as described above, because a compensation TFT 30 isprovided between the second TFTs 24 and the power supply line VL, evenwhen there is a shift in the threshold value of the second TFTs 24 dueto the variations in conditions such as, for example, annealingcondition, the shift can be alleviated by the compensation TFT 30.

[0054]FIG. 7 shows a further example pixel circuit structure accordingto the first embodiment. This circuit structure differs from that shownin FIG. 6 in that not only are the second TFTs 24 provided in plurality,but that the compensation TFTs are also provided in plurality. Eachcompensation TFT 34 is provided between the power supply line VL and thesecond TFTs 24. As shown in FIG. 7, by providing a plurality ofcompensation TFTs 34, variations in the current supply capabilitygenerated among the compensation TFTs 34 can be alleviated and, thus,reliability of reduction in variation in current supply capability tothe organic EL element 50 can be enhanced.

[0055]FIG. 8 shows one example of the planer structure of the organic ELdisplay device having a circuit structure shown in FIG. 7. FIG. 9A is aschematic cross section along the A-A line in FIG. 8, FIG. 9B is aschematic cross section along the B-B line in FIG. 8, and FIG. 9C is aschematic cross section along the C-C line in FIG. 8. In FIGS. 9Athrough 9C, the layers (films) that are simultaneously formed areassigned the same reference numeral except where their functions aredifferent.

[0056] As shown in FIG. 8, each pixel includes a first TFT 10, a storagecapacitor Cs, two pch-second TFTs 24, two nch-compensation TFTs 34 whichare diode-connected between the power supply line VL and the second TFTs24, and an organic EL element 50 connected to the drains of the secondTFTs 24. In the example of FIG. 8, although the configuration is notlimited to the one shown, a pixel is placed in the region surrounded bya gate line GL extending in the row direction and a power supply line VLand a data line DL both extending in the column direction. Also in theexample of FIG. 8, delta arrangement is employed for realizing a morehigh resolution color display device wherein the positions for R, G, andB pixels are shifted at each row, and consequently, the data line DL andthe power supply line VL are not straight, but extend in the columndirection through the gap between pixels having positions shifted foreach row.

[0057] In each of the pixel regions, the first TFT 10 is formed near thecross section between the gate line GL and data line DL. As an activelayer 6, p-Si obtained by polycrystallizing a-Si by a laser annealingprocess is used. The active layer 6 has a pattern where it steps overtwice the gate electrode 2 which protrudes from the gate line GL.Although a single gate structure is shown in FIG. 7, in the circuit, adual gate structure is employed. The active layer 6 is formed on a gateinsulation film 4 which is formed to cover the gate electrode 2. Thesections of the active layer 6 immediately above the gate electrodes 2form channels and source region 6S and drain region 6D to which animpurity is doped are formed around the channels. Because it isdesirable that the first TFT 10 responds quickly to the selection signaloutput on the gate line GL, impurity such as phosphorous (P) is dopedinto the source region 6S and the drain region 6D, to form an nch-TFT.

[0058] The drain region 6D of the first TFT 10 is connected, via acontact hole opened in an interlayer insulation film 14 formed to coverthe entire first TFT 10, to the data line DL formed on top of theinterlayer insulation film 14.

[0059] The source region 6S of the first TFT 10 is connected to thestorage capacitor Cs. The storage capacitor Cs is formed in the regionwhere a first electrode 7 and a second electrode 8 are overlapped withthe gate insulation film 4 in between. The first electrode 7 extends inthe row direction in FIG. 8, similar to the gate line GL, and is formedintegrally with a capacitor line SL formed from the same material as thegate. The second electrode 8 is integral with the active layer(semiconductor layer) 6 of the first TFT 10 and is constructed by theactive layer 6 extending to the formation position of the firstelectrode 7. The second electrode 8 is connected to the gate electrodes25 of the second TFTs 24 via a connector 42.

[0060] The cross sectional structures for the two pch-second TFTs 24 andthe two nch-compensation TFTs 34 are shown in FIG. 9B. The second TFTs24 and the compensation TFTs 34 use a semiconductor layer 16 patternedin an island-like manner for each TFT in the direction along the dataline DL (power supply line VL), as an active layer. Therefore, in thisexample, the channels of the second TFTs 24 and of the compensation TFTs34 are arranged so that the channel length direction is along the dataline DL, that is, along the longitudinal direction of the pixel havingan elongated shape. The semiconductor layer 16 is simultaneously formedwith the active layer 6 of the first TFT 10, and a polycrystallinesilicon formed by polycrystallizing a-Si by a laser annealing process isused as the semiconductor layer 16.

[0061] The compensation TFTs 34 placed at both ends of FIG. 9B areconnected at their respective drain region to the same power supply lineVL via a contact hole opened in the interlayer insulation film 14. Agate electrode 35 is provided immediately below the channel region ofthe compensation TFT 34 with the gate insulation film 4 in between. Thegate electrode 35 is a layer formed by the same material as andsimultaneously with the gate line GL, and is connected to the powersupply line VL at a contact hole, as shown in FIG. 8. Therefore, thecompensation TFTs 34 construct diodes in which both gates and drains areconnected to the power supply line VL, as shown in the circuit diagramof FIG. 7. The source region of each compensation TFT 34 is provided tobe distant from the source region of the second TFT 24 constructed froma pch-TFT, and is connected to the source region of the second TFT 24 bya contact wiring 43.

[0062] Similar to the gate electrode 35 of the compensation TFTs 34,each gate electrode 25 of the second TFTs 24 is a conductive layerformed from the same material as and simultaneously with the gate lineGL. The conductive layer is connected to the second electrode 8 of thestorage capacitor Cs via the connector 42, extends from the formationregion of the storage capacitor Cs along the power supply line VL,extends further under the active layer 16, and forms each of the gateelectrodes 25 of the two second TFTs 24.

[0063] The organic EL element 50 has a cross sectional structure asshown in, for example, FIG. 9C, and is formed on top of a flatteninginsulation layer 18 provided over entire substrate for flattening theupper surface after each of the TFTs are formed as described above. Theorganic EL element 50 is constructed by laminating an organic layerbetween an anode (transparent electrode) 52 and a cathode (metalelectrode) 57 formed at the uppermost layer and common to all pixels.Here, the anode 52 and the source region of the second TFT 24 are notdirectly connected, but are connected via a connector 40 whichconstructs a wiring layer.

[0064] In the first embodiment, as shown in FIG. 8, two second TFTs 24are both connected to a connector 40 and the connector 40 contacts thefirst electrode 52 of the organic EL element 50 at one contact point. Inother words, the organic EL element 50 is connected to n second TFTs 24at contact points having the number equal to or smaller than (n−1).Because the contact region sometimes become a non-illuminating region,by minimizing the number of contact points between the organic ELelement 50 and the connector 40 (second TFTs 24), it is possible tomaximize the illumination region. Another example related to the numberof contacts will be described later as a third embodiment.

[0065] In the first embodiment, as shown in FIGS. 8 and 9C, theconnection position between the connector 40 and the anode 52 isarranged so that it is shifted from the connection position between theconnector 40 and the second TFTs 24. In the emissive element layer 51which will be described later and which includes an organic compound,electric field concentration tends to occur at a locally thin region,and degradation may be caused from the region where electric fieldconcentration occurred. Therefore, it is desirable that the formationregion of the emissive element layer 51 in which an organic material isused be as flat as possible. In the upper layer of a contact hole, arecess due to the contact hole is produced, and the depth of the recessbecomes larger as the contact hole becomes deeper. Therefore, by placingthe contact hole for connecting the connector 40 and the source regionof the second TFTs 24 at a region outside the formation region of theanode 52, it is possible to make the upper surface of the anode 52 ontowhich an organic layer is formed as flat as possible. An example forflattening the upper surface of the anode 52 will be described later toillustrate a fourth embodiment of the present invention.

[0066] The emissive element layer (organic layer) 51 comprises, from theside of the anode, for example, a first hole transport layer 53, asecond hole transport layer 54, an organic emissive layer 55, and anelectron transport layer 56 laminated in that order. As an example, thefirst hole transport layer 52 includesMTDATA:4,4′,4″-tris(3-methylphenylphenylamino)triphenylamine, the secondhole transport layer 54 includesTPD:N,N′-diphenyl-N,N′-di(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine,the organic emissive layer 55 includes, although dependent on the targetillumination color of R, G, and B, for example,BeBq₂:bis(10-hydroxybenzo[h]quinolinato)beryllium which includesquinacridone derivative, and the electron transport layer 56 isconstructed from BeBq. In the example of the organic EL element 50 shownin FIG. 9C, the layers (53, 54, 56, and 57) other than the anode 52constructed from an ITO (indium Tin Oxide) or the like and the organicemissive layer 55 are formed to be common to every pixel. Anotherexample of the structure of the EL element can be constructed bysequentially laminating the layers of (a) transparent layer (anode); (b)a hole transport layer constructed from NBP; (c) an emissive layerincluding red (R) constructed by doping a red dopant (DCJTB) into a hostmaterial (Alq₃), green (G) constructed by doping a green dopant(coumarin 6) into a host material (Alq₃), and blue (B) constructed bydoping a blue dopant (perylene) into a host material (BAlq); (d) anelectron transport layer constructed from Alq₃; (e) an electroninjection layer constructed from lithium fluoride (LiF); and (f)electrode (cathode) constructed from Aluminum (Al). The official namesof the above materials described in abbreviations are as follows:

[0067] “NBP”:N,N′-Di((naphthalene-1-yl)-N,N′-diphenyl-benzidine);

[0068] “Alq₃”:Tris(8-hydroxyquinolinato)aluminum;

[0069]“DCJTB”:(2-(1,1-Dimethlethyl)-6-(2-(2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl)-4H-pyran-4-ylidene)propanedinitrile;

[0070] “coumarin 6”:3-(2-Benzothiazolyl)-7-(diethylamino)coumarin; and

[0071]“BAlq”:(1,1′-Bisphenyl-4-Olato)bis(2-methyl-8-quinolinplate-N1,08)Aluminum.

[0072] The present invention, however, is not limited to theseconfigurations.

[0073] In a pixel having the structure as described above, when aselection signal is applied to the gate line GL, the first TFT 10 isturned on. The potential of the data line DL and the potential of thesource region of the first TFT 10 connected to the second electrode 8 ofthe storage capacitor Cs become equal to each other. A voltagecorresponding to the data signal is supplied to the gate electrode 25 ofthe second TFT 24, and the second TFT 24 supplies, depending on thevoltage value, a current to the anode 52 of the organic EL element 50,which is supplied from the power supply line VL via the compensation TFT34. With this operation, a current based on the data signal can beaccurately supplied to the organic EL element 50 for each pixel and,thus, uniform display without variation can be achieved.

[0074] As shown in FIG. 8, because a plurality of (in this example, two)compensation TFTs 34 and second TFTs 24 are provided between the powersupply line VL and the organic EL element 50 in that order, when acharacteristic shift or deficiency is generated at one of these TFTs dueto a variation, the variation in the supplied amount of current, whichis determined by the total of the plurality of groups, is alleviated dueto the presence of the other TFT having normal characteristics.

[0075] In the planer placement shown in FIG. 8, a polycrystallinesilicon layer produced by polycrystallization by laser annealing processis used as the active layers. The annealing process may be performed,for example, by scanning a laser beam which is longer in the rowdirection of the figure, in the column direction. Even in such a case,the channel direction of the first TFT 10 and the length channeldirection of the active layers of each of the second and compensationTFTs 24 and 34 do not coincide, and the formation positions for thefirst and second TFTs 10 and 24 are far apart. Therefore, it is possibleto prevent simultaneous generation of failures in the first and secondTFTs 10 and 24 and in the second and compensation TFTs 24 and 34 by thelaser annealing.

[0076] In the above, all of the first TFT 10, second TFTs 24, andcompensation TFTs 34 are described as a bottom gate structure, but theseTFTs can have a top gate structure wherein the gate electrode is formedon an upper layer than the active layer.

[0077] As described above, according to the first embodiment, it ispossible to alleviate variations in characteristic of the transistor forsupplying power to an element to be driven such as an organic EL elementand, thus, it is possible to average the variation in the supplied powerto the element to be driven and to prevent variations in illuminationbrightness (luminance) or the like at the element to be driven.

Second Embodiment

[0078] A second embodiment of the present invention will now bedescribed. In the first embodiment, in order to prevent variation in theillumination brightness among pixels as a result of characteristicvariations in the transistor, a compensation thin film transistor havingan opposite conductive characteristic as the element driving thin filmtransistor is provided. In contrast, in the second embodiment, thevariation in the illumination brightness among pixels is inhibited byconsidering the placement of the element driving thin film transistor(second TFT). FIGS. 10A and 10B show an example configuration of onepixel according to the second embodiment. FIG. 10A is a schematic planerview and FIG. 10B is a cross sectional view along the B-B line in FIG.10A. This structure is shown with the same circuit structure as that ofFIG. 1. In these figures, the components corresponding to those in thedrawings that are already explained will be referred to by the samereference numerals.

[0079] In the second embodiment, one pixel comprises an organic ELelement 50, a first TFT (switching thin film transistor) 10, a storagecapacitor Cs, and a second TFT (element driving thin film transistor)20. In contrast to the first embodiment, a single second TFT 20 isformed between the power supply line VL and the organic EL element 50,and the second TFT 20 is placed so that its channel length direction isalong the longitudinal direction of the pixel formed in an elongatedshape, similar to the configuration shown in FIG. 8. In the secondembodiment, by arranging the second TFT 20 so that the channel lengthdirection is along the longitudinal direction of the pixel region, theillumination region of the organic element 50 can be maximized, and, atthe same time, the necessary TFT can be efficiently placed in one pixelregion which has a limited area, even in the case where a second TFT 20having a long channel length is to be placed or in the case where asecond TFT 20 and a compensation TFT 30 must be placed between the powersupply line VL and the organic EL element 50 as shown in FIG. 8.

[0080] In the second embodiment, by placing the second TFT 20 in thelongitudinal direction of the pixel, the channel length of the secondTFT 20 can be lengthened to a sufficient length, as shown in FIGS. 10Aand 10B. By lengthening the channel length of the second TFT 20 to asufficient length, the reliability can be improved because thedurability of the TFT is improved. Moreover, this configuration enablesaveraging of the transistor characteristic of the second TFT 20, and,thus enables reduction in variations in the current supply capability ofthe second TFT 20 among pixels. The reduction of capability variationthen allows for significant reduction of the variation in theillumination brightness of the organic EL element 50 caused by such acapability variation.

[0081] In the second embodiment, the second TFT 20 uses apolycrystalline silicon layer obtained by polycrystallizing an amorphoussilicon layer by laser annealing as the semiconductor layer (activelayer) 16, similar as in the first embodiment. In this case, by settingthe scan direction of the laser annealing to coincide with the channellength direction of the second TFT 20, that is, by placing theirradiating region of the pulse laser so that the edge in thelongitudinal direction crosses in the width direction of the channel 16c and by lengthening the channel length of the second TFT 20 asdescribed above, the characteristics variation in the second TFT 20 canbe reduced because it is easy to adjust the laser so that the entirechannel region of the second TFT 20 is not annealed by a single lasershot, and because generation of a large difference in the characteristicof the second TFT 20 among other pixels can be prevented. Thus, it ispossible to obtain even higher averaging effect on the characteristic ofthe second TFT 20.

[0082] It is desired that the second TFT 20 supplies a relatively largecurrent from the driving power supply (power supply line VL) to theorganic EL element 50. When a p-Si TFT which uses polycrystallinesilicon for the active layer 16 is used for the second TFT 20, themobility of p-Si is sufficient with respect to the desired capability,and, thus, the second TFT 20 can achieve sufficient current supplycapability even when the channel length is designed to be lengthened.Because the second TFT 20 is directly connected to the power supply lineVL, the required durability is high, and consequently, it is oftendesired that the channel length CL be longer than the channel width.Thus, in addition to the above viewpoint, it is desirable that thechannel length of the second TFT 20 be lengthened to a sufficientlength. By forming the second TFT 20 so that the channel lengthdirection is along the longitudinal direction of the pixel region, thesecond TFT 20 with a long channel can be efficiently placed within onepixel region.

[0083] In a display device constructed by arranging a plurality ofpixels on the display surface in a matrix form, the shape of each of thepixels tends to be designed to have a shape that is longer in the columndirection as shown in FIGS. 8 and 10A. In such a case, by placing thesecond TFT 20 so that the channel length direction is along the columndirection, the channel length would be along the longitudinal directionof the pixel region, and thus, the desired channel length as describedabove can be easily secured.

[0084] As shown in the second embodiment, in an active matrix typedisplay device wherein a switching element is provided in each pixel fordriving the display element, a data line DL for supplying a data signalto the first TFT 10 is provided in the column direction and a selectionline (gate line) GL is provided in the row direction. By placing thesecond TFT 20 so that its channel length direction is along theextension direction of the data line DL (column direction), efficientplacement of the second TFT 20 within the pixel region while securing along channel length can be facilitated. In the example shown in FIGS.10A and 10B, a layout wherein the power is supplied from a driving powersupply Pvdd to each pixel by the power supply line VL is employed.Because the power supply line VL also extends in the column directionsimilar to the data line, the channel length direction of the second TFT20 also coincides with the extension direction of the power supply lineVL.

[0085] In the second embodiment, as described above, the second TFT 20is arranged so that its channel length direction coincides with the scandirection of the laser annealing or is parallel to the column direction(extension direction of the data line DL), but the first TFT 10 isplaced so that its channel length direction coincides with the extensiondirection of the gate line GL, that is, the row direction. Thus, in thesecond embodiment, the first TFT 10 and the second TFT 20 have differentchannel length directions.

[0086] A cross sectional structure of the display device according tothe second embodiment will now be described referring to FIG. 10B. FIG.10B shows a cross sectional structure of the second TFT 20 and theorganic EL element 50 which is connected to the second TFT 20. The firstTFT 10, which is not shown, has a basic structure similar to that of thesecond TFT 20 shown in FIG. 10B, with the exceptions that the first TFT10 has a different channel length, a double gate structure, and adifferent conductive type for the active layer 6.

[0087] The first TFT and second TFT shown in the first embodiment bothhave a bottom gate structure, but the first TFT 10 and the second TFT 20of the second embodiment have a top gate structure wherein the gateelectrode is formed on an upper layer than the active layer. Thestructure of the second embodiment is not limited to the top gatestructure, and a bottom gate structure may also be employed.

[0088] The active layer 16 of the second TFT 20 and the active layer 6of the first TFT 10 are both constructed from polycrystalline siliconobtained by laser annealing and polycrystallizing an amorphous siliconlayer formed on a substrate 1, as described above. A gate insulationfilm 4 is formed on top of the active layers 6 and 16 constructed formpolycrystalline silicon. Each of the gate electrodes 2 and 25respectively of the first TFT 10 and of the second TFT 20 is formed onthe gate insulation film 4. The gate electrode 25 of the second TFT 20is connected to the second electrode 8 of the storage capacitor Cs whichis integral with the active layer 6 of the first TFT 10. As shown inFIG. 10A, the gate electrode 25 is patterned so that it extends from theconnection section with the storage capacitor Cs in the column directionand widely covers the section of the gate insulation film 4 above theactive layer 16.

[0089] The region of the active layer 16 of the second TFT 20 which iscovered by the gate electrode 25 at the top is the channel region 16 c.A source region 16 s and a drain region 16 d are formed at both sides ofthe channel region 16 c. In the second embodiment, the source region 16s of the active layer 16 is electrically connected to the power supplyline VL near the storage capacitor Cs via a contact hole formed topenetrate through the gate insulation film 4 and the interlayerinsulation film 14. The drain region 16 d is connected to a connector(wiring layer) 40 near the gate line GL which corresponds to the nextrow of the matrix, via a contact hole formed to penetrate through thegate insulation film 4 and the interlayer insulation film 14. Theconnector 40 extends from the connection region with the drain region 16d to the formation region of the organic EL element 50, and iselectrically connected to an ITO electrode (anode) 52 of the organic ELelement 50 via a contact hole formed on a first flattening insulationlayer (planarizating insulation layer) 18 which is formed to cover theinterlayer insulation film 14, power supply line VL, and connector 40.

[0090] In FIG. 10B, only the central region of formation of the anode 52of the organic EL element is opened above the first flattening layer 18.A second flattening (planarizating) insulation layer 61 is formed tocover the edge of the anode 52, wiring region, and the formation regionsfor the first and second TFTs. The emissive element layer 51 of theorganic EL element 50 is formed on the anode 52 and the secondflattening insulation layer 61. A metal electrode 57 which is common toall pixels is formed on top of the emissive element layer 51.

[0091] The relationship between the channel length CL of the second TFT20 and the moving pitch P of the laser will now be explained. Asdescribed above, it is desired that the channel length CL of the secondTFT 20 be sufficiently long. In order to prevent annealing of the entirechannel region by one pulse laser, it is preferable to set the movingpitch P of the laser and the channel length CL so that P<CL. In somecases, the moving pitch P is adjustable according to the setting of theoptical assembly system of the laser annealing device or the like. Insuch a case, it is preferable that the device be adjusted so that CL>P.In a display device having a resolution of about 200 dpi, for example,even when the length in the pixel row direction is about 30 μm, about 80μm can be secured in the column direction. Moreover, in a configurationwherein the moving pitch P of the laser is between 20 μm and 35 μm, byplacing the second TFT 20 so that its channel length direction is alongthe pixel longitudinal direction, a length of 50 μm to 80 μm can besecured as the channel length CL, and thus, the above relationship canbe satisfied. With such a relationship, the channel region 16 c of thesecond TFT 20 is always polycrystallized by a plurality of pulse laserirradiations, and it is possible to reduce the difference in thecharacteristic from the second TFT 20 of other pixels which aresimilarly polycrystallized by a plurality of pulse laser irradiations.

[0092] In the above explanation, a single second TFT 20 is formedbetween the organic element 50 and the power supply line VL in onepixel. However, the second embodiment is not limited to such aconfiguration, and a plurality of second TFTs 20 may be provided in onepixel. FIG. 11 shows an example layout for a case wherein a plurality ofsecond TFTs 20 are connected in parallel between the power supply lineVL and the organic EL element 50. The equivalent circuit of the pixelstructure shown in FIG. 11 is similar to the case where the compensationTFT 30 is removed from the circuit shown in FIG. 6. The source regions16 sa and 16 sb of two second TFTs 20 are both connected to the powersupply line VL and the drain regions 16 da and 16 db are both connectedto the anode 52 of the organic EL element 50 via a contact 40respectively. By providing a plurality of second TFTs 20 in one pixel inthis manner, the probability that no current can be supplied to theorganic EL element because both of the second TFTs 20 within one pixelsimultaneously became deficient can be reduced, at least to ½ or less.

[0093] The placement of two second TFTs 20 a and 20 b is such that thechannel length direction of the second TFTs 20 a and 20 b isapproximately parallel to the longitudinal direction (in this case, thisdirection coincides with the extension direction of the data line DL) ofthe pixel region similar to FIG. 10A. With such a placement, it ispossible to maximize the illumination region and, at the same time, tosecure maximum length for each channel length CL. The scan direction ofthe laser anneal is set, even in FIG. 11, to be parallel to both channellength directions of the two second TFTs 20 a and 20 b. The activelayers 16 a and 16 b are placed in a straight line. It is not necessarythat the active layers for a plurality of second TFTs 20 a and 20 b beprovided on a straight line, but because the channel regions 16 ca and16 cb of the second TFTs 20 a and 20 b do not completely coincide withthe laser scan direction and are somewhat shifted, it is possible tomore reliably prevent the characteristics of the TFTs 20 a and 20 b tovary in the same manner. In other words, because the channel lengthdirection is shifted from each other in the laser scan direction, theprobability that the channel for the two TFTs are simultaneouslyannealed by the same pulse is reduced and, thus, the probability of aproblem such as, for example, the characteristics of the second TFTs 20a and 20 b being shifted from the set value in the same manner orsimultaneous failure of both transistors can be significantly lowered.Therefore, the variation in the total amount of current supplied to theorganic EL element 50 among the pixels can be reduced.

[0094] It is preferable that both channel lengths CLa and CLb of the twosecond TFTs 20 a and 20 b be greater than the moving pitch P of thelaser, as described above. Moreover, it is preferable that theseparation distance L between the channels 16 ca and 16 cb of theplurality of second TFTs 20 a and 20 b be greater than the moving pitchP of the laser. However, when a plurality of second TFTs 20 is providedin one pixel, as shown in FIG. 11, simultaneous failure in the pluralityof transistors (TFT) 2 a and 2 b within a pixel or characteristic shiftin the same manner can be prevented and, thus, the reduction effect inthe characteristic variation among pixels can be achieved by at leastsetting the sum of the total channel length of the TFTs 20 a and 20 band the separation distance L to be larger than the moving pitch P.

[0095] As described above, according to the second embodiment, it ispossible to alleviate variations in characteristics of the transistorfor supplying power to an element to be driven such as an organic ELelement, and thus, it is possible to average the variation in thesupplied power to the element to be driven and to prevent variations inillumination brightness or the like at the element to be driven.

Third Embodiment

[0096] A method for efficiently connecting a plurality of second TFTs 20and corresponding organic EL element 50 within one pixel will now bedescribed as a third embodiment of the present invention. As describedin the first embodiment and as shown in FIG. 11 of the secondembodiment, provision of a plurality of second TFTs 20 between anorganic EL element 50 and a power supply line VL within one pixel isadvantageous from the viewpoint of improvements in reliability,characteristic, or the like. In a case wherein a plurality of secondTFTs 20 are provided within one pixel, as shown in FIG. 11, byrespectively connecting the second TFTs 20 a and 20 b and the organic ELelement 50, a current can more reliably be supplied from the powersupply line VL to the organic EL element 50 via the second TFTs 20.However, in an organic EL element of the type shown in FIG. 10B in whichlight from the emissive layer 55 is emitted from a transparent anode 52to the outside via the substrate 1 at the lower section, the contactsection usually has a light blocking characteristic. For example, inFIGS. 9C and 10B, the connection between the organic El element 50 andthe second TFT 20 is achieved by the wiring layer 40 (connector) whichis a metal wiring, and at the contact section between the wiring layer40 and the anode 52, the wiring layer 40 having a light blockingcharacteristic is present below the anode 52. Thus, in this region, thelight from the emissive layer 55 cannot pass through to the side of thesubstrate 1. Therefore, if n contact sections are provided between thesecond TFTs 20 and the organic EL element 50 to correspond to the nsecond TFTs 20, the illumination area would be reduced in proportion tothe number of contacts.

[0097] Therefore, in order to minimize the reduction in the illuminationarea, it is preferable to set the number of contacts between the secondTFTs 20 and the organic EL element 50 to be less than or equal to (n−1),wherein the number of second TFTs 20 in one pixel is n (n≧2). In FIG. 8and in FIGS. 12, 13, and 14 to be described below, n second TFTs 20 andthe organic EL element 50 are connected with the number of contactsbeing equal to or less than (n−1). In the figures to be explained below,the components that are common to the figures already described will beassigned the same reference numerals and will not be described again.

[0098]FIG. 12 shows a contact method between second TFTs 20 a and 20 band an organic EL element 50 when two second TFTs 20 a and 20 b areprovided in parallel between the power supply line VL and the organic ELelement 50. Similar to FIG. 11, the two TFTs 20 a and 20 b are placedsuch that respective channel length direction is parallel to thelongitudinal direction of the pixel (the extension direction of the dataline DL) or to the scan direction of laser annealing. The TFTs 20 a and20 b are also placed so that they are shifted from each other. With sucha configuration, as described above, the illumination variation amongpixels can be reduced and the reliability can be improved.

[0099] In the example shown in FIG. 12, a semiconductor layerconstructed from p-Si patterned into a single island-like manner is usedfor the active layers 16 a and 16 b of the second TFTs 20 a and 20 b.The semiconductor is patterned so that both ends in the column directionare the source regions (in the case of a pch-TFT) 16 sa and 16 sb ofrespective second TFTs 20 a and 20 b, and are connected to the powersupply line VL. The region around the center of the semiconductorpattern defines the drain regions (in the case of a pch-TFT) 16 da and16 db of the TFTs 20 a and 20 b, and the drain regions are connected toa single wiring layer 40 provided between the two TFTs via a commoncontact hole formed to penetrate through the interlayer insulation film14 and the gate insulation film 4 (refer to FIG. 10B).

[0100] The wiring layer 40 extends to the anode formation region of theorganic EL element 50. Similar to the cross sectional structure shown inFIG. 10B, the wiring layer 40 is connected to the anode 52 of theorganic EL element 50 via one contact hole opened on the firstflattening insulation layer 18. Here, the connection position betweenthe wiring layer 40 and the anode 52 is set in FIG. 12 to be around thecenter of the anode 52 in the longitudinal direction of the pixel. Thecontact position is not limited to the configuration of FIG. 12, but byplacing the contact position near the center of the anode 52 as shown inFIG. 12, averaging effect of the current density can be obtained in theformation region of the anode 52 which is constructed from an ITO or thelike having a relatively high resistance compared to a metal electrodeand, thus, the uniformity of the illumination brightness at theillumination surface of each pixel can be improved.

[0101] In the example shown in FIG. 13, the number of second TFTs 20 isincreased to three. Three second TFTs 20-1, 20-2, and 20-3 are connectedin parallel between the power supply line VL and the anode of theorganic EL element 50. The active layer 16 of three second TFTs 20 areintegrally formed and are set so that the channel length direction isalong the row direction in FIG. 13. Each of the channel regions 16 c ₁through 16 c ₃ of second TFTs 20-1 through 20-3 are separated in theirchannel width directions by openings in the pattern of the active layer16.

[0102] Here, the three second TFTs 20 are connected to the power supplyline VL at one contact point, and also connected to the anode 52 of theorganic EL element 50 at one contact point by a single wiring layer 40.The gate electrode 25 is common to all three TFTs, is electricallyconnected to the second electrode 8 of the storage capacitor Cs, and isconstructed from a metal wiring extending in the column direction fromaround the storage capacitor Cs. In the configuration example of FIG.13, three second TFTs 20-1 through 20-3 are connected to the organic ELelement 50 by one contact section. Therefore, the ratio of theoccupational area of the contact section within the formation region ofthe organic EL element 50 can be lowered, and thus, the ratio of openingin one pixel, that is, the illumination area, can be increased.

[0103] In an example shown in FIG. 14, the number of second TFTs 20 isincreased to 4. The four TFTs 20-1 through 20-4 are electricallyconnected in parallel between the power supply line VL and the anode 52of the organic EL element 50. The active layer 16 of four second TFTs 20are integrally constructed and the channel length directions of the TFTs20-1 through 20-4 are set to be parallel to the longitudinal directionof the pixel region or the extension direction of the data line DL,similar to FIG. 12. The four TFTs are arranged in an almost straightline.

[0104] Four TFTs 20-1 through 20-4 are connected to the power supplyline VL at three contact points, and connected to the anode 52 of theorganic EL element 50 at two contact points by first and second wiringlayers 40-1 and 40-2. In the example structure shown in FIG. 14, thesource regions 16S₁ and 16S₄ of the TFTs 20-1 and 20-4 which are locatedat the outermost positions of the single active layer 16 arerespectively connected to the power supply line VL as a separate entity.The source regions 16S₂ and 16S₃ of the TFTs 20-2 and 20-3 which arelocated at the central position are together connected to the powersupply line VL. The second TFTs 20-1 and 20-2 and the organic EL element50 are connected as follows. The drain regions 16 d ₁ and 16 d ₂ of thesecond TFTs 20-1 and 20-2 are connected to a first wiring layer 40-1extending from between the second TFTs 20-1 and 20-2 to the element 50,and the first wiring layer 40-1 extends to the formation region of theorganic EL element 50 and is connected to the anode 52 of the element.The second TFTs 20-3 and 20-4 are connected to the organic EL element 50as follows. The drain regions 16 d ₃ and 16 d ₄ of the second TFTs 20-3and 204 are connected to a second wiring layer 40-2 extending frombetween the second TFTs 20-3 and 20-4 to the element 50, and the secondwiring layer 40-2 extends to the formation region of the organic ELelement 50 and is connected to the anode 52 of the element. In thismanner, four second TFTs 20-1 through 20-4 are connected to the organicEL element 50 only at two contact points, in order to inhibit thereduction of illumination region caused by providing four second TFTs20-1 through 20-4.

[0105] In the configuration of FIG. 14, because the four second TFTs20-1 through 20-4 are placed so that the channel length direction isdirected almost in a straight line along the longitudinal direction ofthe pixel, it is possible to efficiently place the second TFTs 20-1through 20-4 within one pixel.

[0106] As described above, according to the third embodiment, byconnecting an element to be driven and a transistor for supplying powerto the element by minimum number of contacts, necessary transistors andelements can be efficiently placed in a limited area. Therefore, when anEL element is used, for example, as the element to be driven, theillumination area ratio can be improved in one pixel unit.

Fourth Embodiment

[0107] A connection structure between the second TFT 20 and the organicEL element 50 will now be described referring to FIGS. 15 through 20. Asdescribed in the third embodiment, in a device in which light istransmitted through a transparent anode 52 and emitted outside from thelower substrate 1 (bottom emission), the contact region between theorganic EL element 50 and the second TFT 20 is usually anon-illuminating region. Also, in order to improve the integrationdensity in many integrated circuits, and in order to improve theresolution in a display device, it is desired to minimize the contactarea. From such a viewpoint, when the active layer 16 of the second TFT20 is directly connected to the anode 52 of the organic EL element 50 orwhen the direct connection is not employed and a metal connection layer(Al layer, Cr layer or the like) is provided in between for improvingthe connection characteristic, it is preferable to form the firstcontact hole 70 of the interlayer insulation film 14 and the secondcontact hole 72 of the first flattening insulation layer 18 to overlapeach other, as shown in FIGS. 15A and 15B.

[0108] However, when a plurality of contact holes are formed to overlapeach other as shown in FIG. 15A, the total step size (h70 +h72) of thecontact holes become large and, thus, the surface flatness of the layerformed on top of the contact hole is reduced. Moreover, there are somecases where a second flattening insulation layer 61 is used for coveringthe edge region of the anode 52 as shown in FIG. 15A in order to preventshortage between the anode 52 and the cathode 57 caused by coveragefailure of the emissive element layer 51 at the anode edge region. Thesecond flattening (planarizating) insulation layer 61 is opened at thecentral region of the anode 52. Therefore, the opened section of thesecond flattening insulation layer 61 is formed near the first andsecond contract holes 70 and 72, and the formation surface of theemissive element layer 51 will be influenced also by the step h74 causedby the opening of the second flattening insulation layer 61.

[0109] In the organic EL element 50, on the other hand, illuminationorganic compound in the emissive layer 55 is illuminated by flowing acurrent through the emissive element layer 51. It is known that if thereis a large difference in the thickness within the layer of the emissiveelement layer 51, an electric field concentration tends to occur at aportion that is thinner than the other portions, and a dark spot tendsto be generated at such a portion. Dark spots degrade display quality,and furthermore, because dark spots tend to expand as the element isdriven, each dark spot shortens the life of the element. Therefore, whenthe organic EL element 50 is formed at a layer above the contact region,it is desired to maximize the flatness of the formation surface of theemissive element layer 51. The contact structure shown in FIGS. 15A and15B in which the emissive element layer 51 is formed on an unevensurface is not desirable from the viewpoint of improving the reliabilityof the emissive element layer 51.

[0110] In consideration of the above, FIGS. 16A and 16B show an exampleof a connection method wherein the flatness at the formation surface ofthe emissive element layer 51 is increased, considering the above. FIG.16A shows a cross sectional structure of the contact section between theactive layer 16 of the second TFT 20 and the anode 52 of the organic ELelement 50. FIG. 16B shows a schematic planer structure of the contactsection. With exception of the presence of the second flatteninginsulation layer 61 for covering the edge region of the anode 52 and thesecond TFT being a top gate structure, the connection structure shown inFIGS. 16A and 16B is identical to the structure shown in FIGS. 8 and 9as explained for the first embodiment. The connection position betweenthe wiring layer 40 and the anode 52 is placed such that it is shiftedwith respect to the connection position between the wiring layer 40 andthe active layer 16 of the second TFT 20. By employing such aconfiguration, the anode surface at the contact region between thewiring layer 40 and the anode 52, being the formation surface of theemissive element layer 51, is only influenced by the step h72 caused bythe second contact hole 72 and is not influenced by the step h70 causedby the first contact hole 70 as in the case shown in FIGS. 15A and 15B.Therefore, as is clear from comparison between FIGS. 15A, B and 16A, B,the flatness of the formation surface for emissive element layer,especially at the illumination region of each pixel onto which theemissive layer 55 is formed can be improved.

[0111]FIG. 17 shows a method for further flattening the formationsurface of the emissive element layer as shown in FIG. 16A. In theexample shown in FIG. 17, similar to FIG. 16A, the formation position ofthe second contact hole 72 for connecting the wiring layer 40 and theanode 52 of the organic EL element 50 is shifted from the formationposition of the first contact hole 70, and, in addition, the secondcontact hole 72 is covered by the second flattening insulation layer 61.Therefore, the region onto which the emissive layer 55 is formed isneither influenced by the step caused by the first contact hole 70 norby the step caused by the second contact hole 72. The flatness of theformation surface of the emissive element layer can thus be furtherimproved. Because the second flattening layer 61 covers the edge regionof the anode 52, shortage between the anode 52 and the cathode 57 or thelike can reliably be prevented.

[0112] The illumination region of the organic EL element is a region inwhich the anode 52 and the cathode 57 oppose each other with theemissive layer 55 in between, and the region in which the secondflattening insulation layer 61 is formed between the anode 52 and theemissive element layer 51 does not illuminate. Therefore, strictlyspeaking, with the configuration shown in FIG. 17, because the secondflattening insulation layer 61 covers not only the edge of the anode 52but also the section above the second contact hole 72, the illuminationregion is reduced. However, as described above, when the wiring layer 40or the like which has a light blocking characteristic is formed at alower layer, the formation region of the wiring layer 40 will be anon-illuminating region when seen from outside. Therefore, even when thestructure shown in FIG. 17 in which the second flattening insulationlayer 61 covers the second contact hole 72 is employed, the actualreduction in the illumination area due to the formation of the secondflattening insulation layer 61 for one pixel can be inhibited.

[0113] The improvement effect on the flatness of the formation surfacefor the emissive element layer can also be achieved by the method ofcovering the contact hole by the second flattening insulation layer 61,by employing a layout wherein the first and second contact holes 70 and72 are placed to overlap each other, as in FIGS. 15A and 15B.Specifically, as in the cross sectional structure of the contact sectionshown in FIG. 18, the active layer 16 of the second TFT 20 and the anode52 of the organic EL element 50 are connected by first and secondcontact holes 70 and 72 formed to overlap each other, and the region ofthe anode 52 having the upper surface recessed because of two contactholes is covered by the second flattening insulation layer 61. Theformation surface for the emissive element layer above the contact holes70 and 72 will thus be a surface with a good flatness, formed by thesecond flattening insulation layer 61. Also, by placing the two contactholes 70 and 72 at the same position in FIG. 18, the element placementefficiency in one pixel is high and it is easy to contribute to animprovement in the illumination region.

[0114]FIG. 19 shows another flattening method of the formation surfacefor the emissive element layer. The method of FIG. 19 differs from thatof FIG. 17 in that a filling layer 62 is selectively formed on top ofthe anode 52 instead of the second flattening insulation layer 61, atthe formation region of the second contact hole 72, in order to fill therecess caused by the contact hole. By selectively forming the fillinglayer 62 on top of the anode 52 for covering the contact hole 72, evenwhen the second flattening insulation layer 61 or the like is notprovided, the formation surface for the emissive element layer above thecontact hole can be flattened. As shown in FIG. 20, the filling layer 62may also be employed, similar to FIG. 19, for a case wherein the firstand second contact holes 70 and 72 are formed to overlap each other. InFIG. 20, the filling layer 62 is selectively formed on the anode 52 atthe region where the two contact holes are formed to overlap, in orderto fill the deep recess formed by the two contact holes. In the casesillustrated in FIGS. 19 and 20, the emissive element layer 51 is formedon a flat surface of the filling layer 62 at the region where thecontact hole or contact holes are formed and, thus, failures in theemissive element layer generating in this region can be prevented.

[0115] The material for the second flattening insulation layer 61 andfor the filling layer 62 can be any material which has a flat uppersurface, but it is preferable to use a stable and insulating materialwhich does not react with the emissive element layer 51 and which is nothydrous. For example, polyimide, HMOSO, TOMCAT, or TEOS can be used.

[0116] As described above, according to the fourth embodiment, theflatness of the surface onto which the element to be driven such as anorganic EL element, is formed can be improved, and thus, it is possibleto improve the reliability of the element to be driven.

What is claimed is:
 1. An active matrix type display device in whicheach of a plurality of pixels arranged in a matrix form comprises atleast an element to be driven and an element driving thin filmtransistor for supplying power from a driving power supply to saidelement to be driven, wherein: each pixel region of said plurality ofpixels has one of the sides in the row and column directions of thematrix longer than the other side; and said element driving thin filmtransistor is placed so that its channel length direction is along thelonger side of said pixel region.
 2. A display device according to claim1, wherein a plurality of said element driving thin film transistors areprovided between said driving power supply and corresponding element tobe driven.
 3. A display device according to claim 1, wherein in saidpixel region, the side along the column direction of the matrix islonger than the side along the row direction of the matrix; and saidelement driving thin film transistor is placed so that its channellength direction is along said column direction.
 4. A display deviceaccording to claim 3, wherein a plurality of said element driving thinfilm transistors are provided between said driving power supply andcorresponding element to be driven.
 5. A display device according toclaim 1, wherein the channel length direction of said element drivingthin film transistor does not coincide with the channel length directionof said switching thin film transistor.
 6. A display device according toclaim 1, wherein said element driving thin film transistor is formed sothat its channel length direction is along the scan direction of a linepulse laser for annealing the channel region of the transistor.
 7. Adisplay device according to claim 6, wherein a plurality of said elementdriving thin film transistors are provided between said driving powersupply and corresponding element to be driven.
 8. A display deviceaccording to claim 1, wherein said element to be driven is an organicelectroluminescence element which uses an organic compound in anemissive layer.
 9. A semiconductor device comprising: at least oneelement driving thin film transistor for supplying driving current froma power supply line to a corresponding element to be driven; and aswitching thin film transistor for controlling said element driving thinfilm transistor based on data supplied when selected; wherein saidelement driving thin film transistor is placed so that its channellength direction is along the extension direction of a data line forsupplying said data signal to said switching thin film transistor.
 10. Asemiconductor device according to claim 9, wherein a plurality of saidelement driving thin film transistors are provided between said drivingpower supply and corresponding element to be driven.
 11. A semiconductordevice according to claim 9, wherein the channel length direction ofsaid element driving thin film transistor does not coincide with thechannel length direction of said switching thin film transistor.
 12. Asemiconductor device according to claim 9, wherein said element drivingthin film transistor is formed so that its channel length direction isalong the scan direction of a line pulse laser for annealing the channelregion of the transistor.
 13. A semiconductor device according to claim12, wherein a plurality of said element driving thin film transistorsare provided between said driving power supply and corresponding elementto be driven.
 14. A semiconductor device according to claim 9, whereinsaid element to be driven is an organic electroluminescence elementwhich uses an organic compound in an emissive layer.